The AddonManager.shutdown barrier blocks the final phase of AddonManager
shutdown, which means that things which use it to mean "I need to do this
before the add-on manager shuts down" are generally broken.

This patch adds a separate beforeShutdown barrier which blocks the start of
provider shutdown, and changes existing broken consumers to use it. It also
renames the existing barrier to `AddonManagerPrivate.finalShutdown` to make it
clearer a) what it does, and b) that only internal AddonManager code should
use it.

Differential Revision: https://phabricator.services.mozilla.com/D33195

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extra : rebase_source : de20ecdc145a3b0c2565da910be6c99698b0b7df
extra : source : 678144498f35fd4ffafe8f0ddf5f64143d380dcb